Treating sapphire objects of irregular shapes



R. T. LUEDEMAN ING SAPPHIRE OBJECTS OF IRREGULAR SHAPES Get. 7 i95 TREATFiled Sept. 22, 1955 ROBERT 7'. LUEDEMA/V INVENTOR.

United States Patent TREATING SAPPHIRE OBJECTS OF IRREGULAR SHAPESRobert T. Luedeman, Leonia, N. J., assignor, by mesne assignments, toDaystrom, Incorporated, Murray Hill, N. J., a corporation of New JerseyApplication September 22, 1955, Serial No. 535,917 3 Claims. (Cl. 49-77)This invention relates to the manufacture of sapphire objects ofirregular shapes and, more particularly, to the manufacture of jewelbearings.

Forces due to surface tension, as present in non-metallic materials whenheated to a temperature near their melting points, have been used tosmooth out surface irregularities in connection with glass fabrication.However, glass, in contrast to crystalline materials such as sapphire,has an extremely wide plastic range and no true melting point.Therefore, control during the heating cycle and the method of heatingare not at all critical. The high melting point of sapphire, which isabout 3742 F., and the necessity of heating to temperatures below butstill very close to that melting point, but not closer than about 100 F.and not further removed than about 600 F., in order that the surfacetension forces may operate effectively, restricts the available means ofheating and makes close control mandatory.

Flame polishing has been used, at relatively uncontrolled temperaturesof 3100 F. to 3650 F., for simple objects. However, on irregularobjects, such as instrument jewel bearings, the action of the flameproduces ripples and other surface distortion, thus usually renderingthe part unsuitable for its intended use. The normal flame-fired,boule-annealing furnaces are also unsatisfactory because of poortemperature control and the possibility of contamination by particles ofrefractories used to line them.

In order to remedy these defects, I propose to firepolish by heating thesapphire objects in a vacuum, either by induction methods using ametallic radiating partscontainer, or by resistance elements. Vacuum isimportant in that it acts as an inert thermally non-conductingatmosphere. This permits the furnace to be constructed with norefractories in the hot zone, using readily controllable electric power,and with the parts not subject to high velocity gas streams.

An object of this invention is to improve on the fire polishing ofcrystallate materials, such as sapphire, in order to avoid surfacedistortion such as rippling.

An object of this invention is to manufacture sapphire V jewels, inwhich the final step is fire polishing in a vacuum at a temperaturebelow but very near the melting point of the sapphire.

An object of this invention is the production of bearing jewelsparticularly adapted for use in electrical instruments, in which theconcave or interior conical surface which receives the rotating bearingis fire polished, after the usual finishing with abrasives, to anextraordinary degree of smoothness, better than that produced by thefinest abrasives, without rippling or other distortion.

These and other objects and advantages will become apparent from thefollowing detailed description when taken with the accompanyingdrawings. It will be understood that the drawings are for purposes ofillustration and do not define the scope or limits of the invention,

reference being had for the pended claims.

In the drawings, wherein like reference characters denote like parts inthe several views:

Figure 1 is an enlarged plan view of a fire polished sapphire bearingembodying my invention;

Figure 2 is a sectional view on the line II-II of Figure l, in thedirection of the arrows;

Figure 3 is a view corresponding to Figure 2, but showing the jewelbearing mounted for use in an electrical instrument; and

Figure 4 is a vertical sectional view of apparatus including a vacuuminduction furnace, which may be used for treating irregular sapphireobjects in accordance with my invention.

Referring to the drawings, there is shown in Figures 1 and 2,considerably enlarged, a sapphire bearing 11 which is generallycylindrical on its outer lateral surface, fiat on its lower surface, andhas a concave, generally-conical surface formed as in indentation 12 inits upper surface. The vertex of the conical surface 12 is not exactly apoint, but is spherical on a very small radius, such as 2 or 3 mils. Thebearing spindle, or pivot, which is to be received in this cavity 12,has its end correspondingly formed, except that the tip is to a somewhatsmaller radius, such as /2 mil less, and the angle of its cone issomewhat more acute. It is necessary that the bearing surface providedin the jewel be very smooth and very accurately formed, so that not onlyis friction minimized, but shifting of the axis of pivoting prevented.

If made of artificial sapphire, the part 11 may be cut from the rawstock or boule and shaped by known methods to take the form illustratedin Figures 1 and 2. After this, it is polished by abrasives, a very fineabrasive being last used, so as to produce a bearing surface as smoothas possible by such method. Finally, a quantity of such instrument cupjewels are placed in a refractory metal cup or crucible 13. Such acrucible may consist of such material as molybdenum, tungsten, or

latter purpose to the ap- 'other high-melting point metal.

The crucible is supported inside of a desirably transparent refractoryenclosure or bell jar 14 which may consist hard glass or Vycor, which is96% silica, desirably in the form of a cylinder with an open bottom endand closed at its top by a spherical portion 15. The enclosure 14 restson a metal plate 16 or other support and is vacuum-sealed thereto as bymeans of wax or other suitable material 17. The crucible 13 is supportedabove the plate 16, as on a suitable hollow refractory material support18, and the air inside the bell jar 14 may be evacuated by means of asuitable pump through the tube 19. Surrounding the enclosure 14 is acoil of wire 21 connected to a suitable source 22 of high-frequencypower through a switch 23.

The apparatus described is operated by placing the sapphire parts in thecrucible or cup 13, supported as illustrated in Figure 4, and a vacuumproduced inside of the enclosure or hell jar 14. After production of ahigh vacuum, power is introduced to the coil 21, as by closing switch23, thereby inducing circulating currents in the crucible l3 and raisingits temperature desirably to between 3600 and 3620 F. Such a temperatureis maintained for about 20 minutes, whereupon the power is cut off andthe apparatus allowed to cool. After this, the vacuum may be relieved,making it possible to remove the bell jar 14 and take out the jewelswhich have been treated. Such a treatment is designed to produce themaximum smoothing of minute imperfections with minimum gross dimensionalchange. The temperatures and times may be increased with increasing sizeof the irregularities to be reduced, the particular treatment used whatlower, such as 3690" F., because above a certain temperature,.dependenton the heating cycle ofthe part, the surface becomes rough andchanneled. This is'probably related to the temperaturejgradients throughthe pieceythe relative fluidity and volatility of the surface layer, andthe crystalline nature off the material. In general, I have found thatbest results are obtained with a relatively low temperature and arelatively long treatment time. However, the lower limit because ofpractical considerations appears to be within the range of 3200 F. to34.00" F. After the treatment has been coneluded, the rateof cooling isnot critical but should be low enough to avoid thermal shock.

Figure 3 shows one of the jewels 11, produced in accordance with my.invention, mounted in metal part 24. The'peripheral portion thereof isheld in place in a cavity 25 by bending over the upper edge of the part24, as indicated at 28. The part of the holding device 24 below thecavity may be threaded, as indicated at 29, for mounting it in theinstrument mechanism in accordance with conventional practice.

Although a preferred embodiment only has been disclosed, it will beunderstood that modifications may be made, not only in the form ofsapphire pieces being treated, but in the form of the apparatus used inthe treatment. For example, the heating may be effected by a coilmounted within the bell jar 14, instead of without, thereby producing abetter coupling. Resistance heating, rather than inductiveheating, maybe employed if desired.

Having now described my invention in detail in accordance with thepatent statutes, those skilled in this art will have no difliculty inmaking changes and modifications in the individual parts or theirrelative assembly in order to meet specific requirements or conditions.

Such changes and modifications may be made without departing from thescope and spirit of my invention, as set forth in the following claims.

Iclaim:

1. The method of forming a smooth wall defining a relatively deep cavityin an instrument bearing made of a crystalline material, said methodcomprising abrasively polishing the c'avity'to' predetermined dimensionsand then heating the bearing in a vacuum to a temperature of l00-600 F.lower than the melting point of the material, the time during which thebearing remains so heated being insufiicient to produce a. dimensionalchange in the said cavity.

2. The invention as recited in claim 1 wherein the bearing material issapphire and the cavity is conical.

3. The invention as recited in claim 2, including the step of coolingthe bearing at a rate slow enough to avoid thermal shock.

References ('Iited in the file of this'patent UNITED STATES PATENTS1,422,216 McDougal et al. July 11, 1922 1,537,036 Miller May 5, 19251,621,446 Watson Mar. 15, 1927 2,048,556 McArthur Iuly 21, 1936 2,398,382 Lyon Apr. 16, 1946 2,405,892 Lederer et al. Aug. 13, 19462,448,511 Barnes et-al. Sept. 7, 1948 2,485,553 Barnes et a1 Oct. 25,1949 2,511,216 Miller June 13, 1950 2,546,002 Jelinek Mar. 20, 19512,597,469 Gatzka May 20, 1952 2,608,031 Barnesetial Aug. 26, 19522,641,954 Scharf et al.. June 16, 1953 2,659,181 Yenni et a1 Nov. 17,1953 2,735,421 Cook Feb. 21, 1956 FOREIGN PATENTS 687,159 Great BritainFeb. 11, 1953

